Method for forming patterns on a semiconductor device using a lift off technique

ABSTRACT

Provided is a technique of improving the properties of a bipolar transistor. Described specifically, upon formation of a collector electrode around a base mesa by the lift-off method, a resist film is formed over connection portions between the outer periphery of a region OA 1  and a region in which the base mesa  4   a  is formed, followed by successive formation of gold germanium (AuGe), nickel (Ni) and Au in the order of mention over the entire surface of a substrate so that the stacked film of them will not become an isolated pattern. As a result, the stacked film over the base mesa  4   a  is connected to a stacked film at the outer periphery of the region OA 1 , facilitating peeling of the stacked film over the base mesa  4   a . In addition, generation of side etching upon formation of a via hole extending from the back side of the substrate to a backside via electrode is reduced by forming the backside via electrode using a material such as WSi which hardly reacts with an n type GaAs layer or n type InGaAs layer.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation of application Ser. No.11/257,060, filed Oct. 25, 2005 now U.S. Pat. 7,214,558, which, in turn,is a parent application of Ser. No. 10/809,525, filed Mar. 26, 2004 (nowU.S. Pat. No. 7,029,938), which claims priority from Japanese patentapplication JP 2003-084220, filed on Mar. 26, 2003, the content of whichis hereby incorporated by reference into this application.

BACKGROUND OF THE INVENTION

The present invention relates to a semiconductor device and to a methodof manufacture thereof; and, more particularly, the invention relates toa technique that is effective when applied to a semiconductor devicewhich is formed by a method having a step of forming patterns by theso-called liftoff method.

There are, for example, semiconductor devices using a Group III-Vcompound of the periodic table, such as gallium arsenide (GaAs). Suchcompound semiconductors are characterized by the fact that they exhibita greater mobility than Si (silicon), which enables preparation ofsemi-insulating crystals and liquid crystals, and a heterojunction canbe formed by using them.

For example, a hetero-junction bipolar transistor (HBT) using galliumarsenide is formed as a bipolar transistor using GaAs as a base layerand a heterogeneous semiconductor, such as InGaP (indium galliumphosphide) or AlGaAs (aluminum gallium arsenide), as an emitter layer.It is possible to increase the a current amplification ratio and therebyimprove the properties of a transistor by using a heterojunction(heterogeneous junction) to increase the forbidden band width of theemitter over that of the base in an emitter-base junction.

For example, Japanese Patent Application Laid-Open No. 2001-1898319([0034], FIG. 4) describes an HBT using GaAs as a base layer and InGaPas an emitter layer. In this device, the emitter contact layer (16) isformed as ;a substantially annular structure. In addition, the emitterelectrode (17) is designed so that wiring does not need to cross overthe emitter electrode (17) upon formation of an extraction interconnectof the base electrode (19).

Japanese Patent Application Laid-Open No. 2002-246587 ([0041], FIG. 17)discloses a GaAs type HBT whose base layer (3) and an emitter layer (4)have a circular planar shape.

Japanese Patent Application Laid-Open No. 2000-277530 (Summary, FIG. 1)discloses a GaAs type HBT. It has, on the back side of its substrate, avia hole and a metal film that is adhered on the back side of thesubstrate.

SUMMARY OF THE INVENTION

The present inventors have carried out an investigation of an HBT usingGaAs. They have studied and developed an HBT, for example, using n typeGaAs for a collector region, p type GaAs for a base region and n typeInGaP for an emitter region.

Over these regions, electrodes (interconnects) for the extraction ofthese regions are formed. Such electrodes are sometimes formed byetching, but when gold (Au) is used as an electrode, patterns are formedby the lift-off method because Au is a material which cannot beprocessed (etched) easily. There are not very many gases or liquids thatare usable for chemical etching of some metals, such as Au, while forsuch metals, a sufficient etching selectivity to an underlying layercannot be secured, which makes physical etching of such a metaldifficult.

The lift-off method is a method of forming a photoresist film in aregion other than a region in which the patterns are to be formed,forming a desired film over the entire surface, and removing thephotoresist film and at the same time, removing the film lying thereoverto leave only the patterns in the region. This method enables processingof metals for which there are no suitable etchants (etching gases) orthose for which sufficient etching selectivity to an underlying filmcannot be secured.

As a result of investigation, however, the present inventors have founda problem in that the film to be removed remains and desired patternscannot be formed. This problem will be described later more specificallywith reference to FIGS. 24 and 25.

In an HBT of the type investigated by the present inventors, an emitterextraction electrode is electrically connected to a back electrode of asemiconductor substrate. Upon electrical connection, in order tominimize the inductance component of the emitter extraction electrode,via holes reaching the emitter extraction electrode are formed from thebackside of the semiconductor substrate, and then, an electrode isformed on the backside of the semiconductor substrate including the viaholes.

The present inventors have found, however, that side etching occurs uponformation of the via holes. On the etched side portions, back electrodesare hardly formed and voids (hollows or gaps) appear. This deterioratesthe adhesion of the back electrode, becoming a cause of peeling.

In addition, when side etching appears, the back electrode at this placebecomes thin and disconnection tends to occur. Even if disconnectiondoes not occur, the wiring resistance rises or the electromigrationresistance lowers, leading to a deterioration in the reliability of theback electrode.

An object of the present invention is to provide a technique forimproving the properties of a bipolar transistor.

The above-described and the other objects, and novel features of thepresent invention will be apparent from the description herein and theaccompanying drawings.

Of the embodiments and features disclosed in the present application,representative examples will next be summarized briefly.

A method of manufacture of a semiconductor device according to theinvention comprises the step of: (a) forming a pattern in a secondregion encompassing therewith a first region over a semiconductorsubstrate, the step (a) including the steps of: (b) forming a first filmover the first region, a third region encompassing therewith the secondregion and first and second connection portions for connecting the firstregion and the third region; (c) after the step (b), forming a secondfilm over the semiconductor substrate; and (d) after the step (c),removing the first film to remove the second film over the first region,the third region and the first and second connection portions andthereby forming two patterns made of the second film over the secondregion.

A semiconductor device according to the present invention comprises: (a)a semiconductor substrate having a first region, a second regionencompassing the first region therewith, and a third region encompassingthe second region therewith; (b) a collector layer formed in the firstregion and the second region; (c) a base layer formed in the firstregion over the collector layer; (d) an emitter layer formed over thebase layer; and (e) a collector electrode formed in the second regionover the collector layer and having a first portion and a second portionseparated from each other by two cutout portions.

A semiconductor device according to the present invention comprises: (a)a substrate having a first main surface and a second main surfaceopposite thereto; (b) a compound semiconductor layer formed over thefirst main surface; (c) a first conductive film formed over the compoundsemiconductor layer; (d) an opening portion extending from the secondmain surface and reaching the first conductive film; and (e) a secondconductive film formed over the second main surface and in the openingportion, wherein the first conductive film is made of a refractory metalor a nitride or silicide of a refractory metal.

A method of manufacture of a semiconductor device according to thepresent invention comprises the steps of: (a) preparing a substratehaving a first main surface and a second main surface opposite thereto;(b) forming a compound semiconductor layer over the first main surface;(c) forming, over the compound semiconductor layer, a first conductivefilm made of a refractory metal, or a nitride or silicide thereof; (d)forming an opening portion extending from the second main surface andreaching the first conductive film; and (e) forming a second conductivefilm over the second main surface and in the opening portion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fragmentary cross-sectional view of a substrate illustratinga manufacturing method of a semiconductor device (HBT) according to oneembodiment of the present invention;

FIG. 2 is a fragmentary plan view of a substrate illustrating themanufacturing method of a semiconductor device (HBT) according to theone embodiment of the present invention;

FIG. 3 is a fragmentary cross-sectional view of a substrate illustratingthe manufacturing method of a semiconductor device (HBT) according tothe one embodiment of the present invention;

FIG. 4 is a fragmentary cross-sectional view of a substrate illustratingthe manufacturing method of a semiconductor device (HBT) according tothe one embodiment of the present invention;

FIG. 5 is a fragmentary cross-sectional view of a substrate illustratingthe manufacturing method of a semiconductor device (HBT) according tothe one embodiment of the present invention;

FIG. 6 is a fragmentary plan view of a substrate illustrating themanufacturing method of a semiconductor device (HBT) according to theone embodiment of the present invention;

FIG. 7 is a fragmentary cross-sectional view of a substrate illustratingthe manufacturing method of a semiconductor device (HBT) according tothe one embodiment of the present invention;

FIG. 8 is a fragmentary plan view of a substrate illustrating themanufacturing method of a semiconductor device (HBT) according to theone embodiment of the present invention;

FIG. 9 is a fragmentary cross-sectional view of a substrate illustratingthe manufacturing method of a semiconductor device (HBT) according tothe one embodiment of the present invention;

FIG. 10 is a fragmentary plan view of a substrate illustrating themanufacturing method of a semiconductor device (HBT) according to theone embodiment of the present invention;

FIG. 11 is a fragmentary cross-sectional view of a substrateillustrating the manufacturing method of a semiconductor device (HBT)according to the one embodiment of the present invention;

FIG. 12 is a fragmentary cross-sectional view of a substrateillustrating the manufacturing method of a semiconductor device (HBT)according to the one embodiment of the present invention;

FIG. 13 is a fragmentary plan view of a substrate illustrating themanufacturing method of a semiconductor device (HBT) according to theone embodiment of the present invention;

FIG. 14 is a fragmentary plan view of a substrate illustrating themanufacturing method of a semiconductor device (HBT) according to theone embodiment of the present invention;

FIG. 15 is a fragmentary plan view of a substrate illustrating themanufacturing method of a semiconductor device (HBT) according to theone embodiment of the present invention;

FIG. 16 is a fragmentary cross-sectional view of a substrateillustrating the manufacturing method of a semiconductor device (HBT)according to the one embodiment of the present invention;

FIG. 17 is a fragmentary cross-sectional view of a substrateillustrating the manufacturing method of a semiconductor device (HBT)according to the one embodiment of the present invention;

FIG. 18 is a fragmentary plan view of a substrate illustrating themanufacturing method of a semiconductor device (HBT) according to theone embodiment of the present invention;

FIG. 19 is a fragmentary cross-sectional view of a substrateillustrating the manufacturing method of a semiconductor device (HBT)according to the one embodiment of the present invention;

FIG. 20 is a fragmentary plan view of a substrate illustrating themanufacturing method of a semiconductor device (HBT) according to theone embodiment of the present invention;

FIG. 21 is a fragmentary plan view of a substrate illustrating themanufacturing method of a semiconductor device (HBT) according to theone embodiment of the present invention;

FIG. 22 is a fragmentary cross-sectional view of a substrateillustrating the manufacturing method of a semiconductor device (HBT)according to the one embodiment of the present invention;

FIG. 23 is a fragmentary plan view of a substrate illustrating themanufacturing method of a semiconductor device (HBT) for explaining theadvantage of the present invention;

FIG. 24 is a fragmentary plan view of a substrate illustrating themanufacturing method of a semiconductor device (HBT) for explaining theadvantage of the present invention;

FIG. 25 is a fragmentary plan view of a substrate illustrating themanufacturing method of a semiconductor device (HBT) for explaining theadvantage of the present invention;

FIG. 26 is a fragmentary cross-sectional view of a substrateillustrating the manufacturing method of a semiconductor device (HBT)for explaining the advantage of the present invention;

FIG. 27 is a fragmentary cross-sectional view of a substrateillustrating the manufacturing method of a semiconductor device (HBT)for explaining the advantage of the present invention;

FIG. 28 is a fragmentary plan view of a substrate illustrating anothersemiconductor device (HBT) according to one embodiment of the presentinvention; and

FIG. 29 is a fragmentary plan view of a substrate illustrating anothersemiconductor device (HBT) according to one embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the present invention will be described hereinafterbased on the accompanying drawings. In all of the drawings, elementshaving a like function will be identified by like reference numerals andoverlapping descriptions thereof will be omitted.

The structure and steps in the manufacture of the semiconductor device(HBT) according to this embodiment will be described with reference toFIGS. 1 to 22, which are fragmentary cross-sectional or fragmentary planviews illustrating the method of manufacture of the semiconductor deviceof this embodiment.

First, the structure of the semiconductor device (HBT) of thisembodiment will be described. The structure will become more apparentfrom the later description of the manufacturing method, so that only thecharacteristic structure will be explained here.

As illustrated in FIGS. 14 and 16, the semiconductor device (HBT) ofthis embodiment has a base mesa 4 a which has the form of a circle witha portion cut out of it. This base mesa 4 a is made of a p type GaAslayer. An n type InGaP (emitter layer) 5 lies over this base mesa 4 a,while an n type GaAs layer (collector layer) 3 lies below the base mesa4 a.

The base mesa 4 a has, at the center thereof, a base electrode 8; and,over the n type InGaP (emitter layer) 5, an upper emitter layer 6 and anemitter electrode 7 are formed so as to surround the base electrode 8.

Around the base mesa 4 a, a collector electrode 9 a made of gold (Au) isformed. This collector electrode 9 a is electrically connected to the ntype GaAs layer (collector layer) 3 via an n⁺ type GaAs layer(sub-collector layer). A combination of this n type GaAs layer(collector layer) 3 and n⁺ type GaAs layer (sub-collector layer) 2 canbe regarded as a collector layer.

What is characteristic here is that the pattern of the collectorelectrode 9 a does not completely encompass the base mesa 4 a, but isformed of a pair of members consisting of a first portion and a secondportion separated by two cutout portions 20 a,20 b. These cutoutportions 20 a,20 b each has a width of about 4 μm in the X direction.The width of each of these cutout portions 20 a, 20 b in the X directionis smaller than the maximum width of the base mesa 4 a in the Xdirection. The first portion has a substantially C-shaped pattern, whilethat of the second portion has an inverted C-shaped pattern. In otherwords, the collector electrode 9 a has a pair of members consisting of aportion extending in the Y direction and a portion extending in the Xdirection from the opposite ends on the side of the pattern in the Ydirection. The pattern consisting of the first portion and the secondportion is electrically connected by a collector extraction interconnectM1 c (refer to FIGS. 17 and 18). The width of each of these cutoutportions 20 a, 20 b may be approximately the minimum processing size(width limit of resolution by photolithography). Such narrowing of thewidth of the cutout portions 20 a, 20 b in the X direction widens thecollector electrode 9 a, leading to an improvement in the properties ofthe device.

These cutout portions 20 a, 20 b are disposed almost symmetrical to thebase mesa 4 a. A base extraction interconnect M1 b is formed over one ofthese cutout portions 20 a, 20 b via an insulating film (refer to FIG.18).

As illustrated in FIG. 22, the backside via electrode 7 v exists in thesame layer with the emitter electrode 7 and a via hole VH reaches afirst-level interconnect M1 v over the backside via electrode 7 v. Onthe back side of the semi-insulating GaAs substrate 1, including theinside of the via hole VH, a back electrode (backside interconnect) 40is formed.

The semiconductor device (HBT) of this embodiment will be described nextin accordance with its manufacturing steps.

As illustrated in FIG. 1, an n⁺ type GaAs layer (sub-collector layer) 2,which is about 700 nm thick, is allowed to grow over a semi-insulatingGaAs substrate (which will hereinafter simply be called a “substrate”),which is about 600 μm thick, by the metal organic chemical vapordeposition method (MOCVD). Over the sub-collector layer, an n type GaAslayer (collector layer) 3, which is about 700 nm thick, and a p typeGaAs layer (base layer) 4, which is about 100 nm thick, are formedsuccessively by the MOCVD method.

An n type InGaP layer (emitter layer), which is 5 about 35 nm thick, isdeposited by the MOCVD method, followed by the formation thereover of anupper emitter layer 6 having a thickness of 400 nm. This upper emitterlayer 6 is made of a stacked film consisting of an n type GaAs layer andan n type InGaAs layer formed thereover. The n type InGaAs layer in theupper emitter layer 6 is used for forming an ohmic contact with anemitter electrode 7, which will be described later.

As described above, semiconductors different in kind (hetero junction)are used for the base layer (p type GaAs layer) 4 and the emitter layer(n type InGaP) 5.

A conductive film, for example, a tungsten silicide (WSi) film, which isabout 300 nm thick, is deposited by sputtering. The Wsi film thus formedis then processed by photolithography and dry etching, whereby anemitter electrode 7 and backside via electrode 7 v are formed.

FIG. 2 is a fragmentary plan view of the emitter electrode 7 after itsformation. As illustrated in FIG. 2, the emitter electrode 7 has apattern formed by an arc and a cord but lacking its center portion. Inother words, it has an imperfect doughnut shape. In a substantiallycircular region in which the emitter electrode 7 is not formed, a baseelectrode is formed. A dashed line defining a rectangular shape in FIG.2 indicates a region in which one HBT is to be formed. FIG. 2 includesonly two HBT formation regions; however, as illustrated in FIG. 21, aplurality of blocks in which a HBT is to be formed exist, and thebackside via electrode 7 v is formed between these blocks.

As illustrated in FIG. 3, using the emitter electrode 7 and backside viaelectrode 7 v as masks, the upper emitter layer 6 is wet etched toexpose the n type InGaP layer (emitter layer) 5. The p type GaAs layer(base layer) 4 may be exposed by etching of the n type InGaP layer(emitter layer) 5.

As illustrated in FIG. 4, a base electrode 8 made of a stacked film ofplatinum (Pt), titanium (Ti), molybdenum (Mo), Ti and gold (Au), whichis stacked in the order of mention, is formed. Its thickness is, forexample, about 300 nm. This base electrode 8 can be formed, for example,by the lift-off method. The lift-off method will be described morespecifically later. By heat treatment (alloying), the Pt lying on thebottom of the base electrode 8 is caused to react with the n type InGaPlayer (emitter layer) and p type GaAs layer (base layer) 4. By thisreaction, ohmic contact between the base electrode 8 and the p type GaAslayer (base layer) 4 can be formed.

As illustrated in FIG. 5, the n type InGaP layer (emitter layer) 5 and ptype GaAs layer (base layer) 4 are etched by a photolithography and wetetching technique, whereby a base mesa 4 a is formed. BMA in thisdiagram indicates the formation region of the base mesa 4 a. As anetchant, a mixed aqueous solution of phosphoric acid and hydrogenperoxide is used, for example. By this etching for separation, eachtransistor has its own n type InGaP layer (emitter layer) 5 and basemesa 4 a.

The formation region (BMA) of the base mesa 4 a is, as illustrated inFIG. 6, a circle with a portion cut out. In other words, its shape isdefined by an arc having a center angle of 180 degree or greater and acord connecting opposite ends of the arc. A region other than thecentral portion (base electrode 8) becomes a pn junction portion betweenthe n type InGaP layer (emitter layer) 5 and p type GaAs layer (baselayer) 4.

From the viewpoint of high frequency properties, the junction capacityCbc between the base layer and the collector layer is preferably smallerrelative to the area of the same emitter layer. In other words, theformation region of the base mesa which is smaller relative to the areaof the same emitter layer is preferred.

As in this embodiment, by forming the base mesa 4 a to havesubstantially the same outer circumference with that of the emitterlayer 5, the formation region of the base mesa 4 a can be made smallerthan that of the emitter layer 5, resulting in a lowering of thejunction capacity Cbc.

The base electrode 8 is located over almost the central portion of thisbase mesa 4 a, and the emitter electrode 7 (upper emitter layer 6)exists at the outer periphery of the base electrode 8.

Upon formation of the base mesa 4 a, the n type InGaP layer (emitterlayer) 5 and p type GaAs layer (base layer) 4 are also removed byetching from the periphery of the backside via electrode 7 v.

Upon etching of the p type GaAs layer (base layer) 4 and the like, theunderlying n type GaAs layer (collector layer) 3 is also etched by about300 nm.

As illustrated in FIG. 7, an insulating film (for example, a siliconoxide film) 13 a, which is about 100 nm thick, is deposited over thesubstrate 1. This insulating film 13 a serves to protect the baseelectrode 8, but the formation of it may be omitted.

By selectively etching the insulating film 13 a and n type GaAs layer(collector layer) 3, the n⁺ type GaAs layer (sub-collector layer) 2 ispartially exposed. This exposed region is defined as OA1. FIG. 8 is afragmentary plan view of the region OA1 after its formation.

A step of forming a collector electrode in this region OA1 by thelift-off method will be described next.

As illustrated in FIG. 9, a photoresist film (which will hereinaftersimply be called a “resist film”) R is formed over the entire surface ofthe substrate 1, and the resist film R over the region OA1 is thenremoved by photolithography. As a result, the n⁺ GaAs layer(sub-collector layer) 2 is exposed from the region OA1. An openingportion OA2 of the resist film R is formed so as to be smaller than theregion OA1 (FIG. 10). In other words, the resist film R is caused tohang over the end portions of the insulating film 13 a or the n typeGaAs layer (collector layer) 3 underlying the resist film R (like anoverhang). The resist film R may be formed to have an inverse taperedshape.

As illustrated in FIGS. 11 to 13, gold germanium (AuGe), nickel (Ni) andAu are stacked successively one after another in the order of mentionover the entire surface of the substrate 1 to form stacked films 9,9 a.FIG. 12 is a partially enlarged view illustrating the vicinity of theregion OA1, while FIG. 13 is a fragmentary plan view illustrating thestacked films 9,9 a after formation.

As illustrated in this diagram, the stacked films 9 and 9 a are formedover the resist film R and in the opening portion OA2, respectively. Theresist film R is formed to hang over so that over the side walls of theinsulating film 13 a or n type GaAs layer (collector layer) 3, thestacked film 9 is not deposited. The lower surface of the resist film Ris exposed from the end portion of the insulating film 13 a.

The resist film R is then removed by a peeling solution (etchant). Theetcharit penetrates from the exposed part of the lower surface of theresist film R and dissolves the resist film R (FIG. 12). When the resistfilm R is removed in such a manner, the stacked film 9 thereover is alsoremoved by peeling. The stacked film remains only inside of the openingportion OA2 (over the region OA1) and becomes a collector electrode 9 a.The fragmentary plan view of the collector electrode 9 a after itsformation is illustrated in FIG. 14, and a fragmentary cross-sectionalview thereof is illustrated in FIG. 16.

What is important here is that two cutout portions (pattern lackingportions 20 a, 20 b) are disposed in the pattern of the collectorelectrode 9 a. These cutout portions (20 a, 20 b) may be regarded as aconnection portion of the resist film R (stacked film 9) (refer to FIGS.10 and 13). As a result of the formation of the resist film R (stackedfilm 9) over the connection portions (20 a, 20 b) between the peripheralportions (third region) in the region OA1 and a region (first region) inwhich the base mesa 4 a has been formed, the cutout portions are formed.The pattern of the collector electrode 9 a is separated by these cutoutportions and is formed of two patterns, that is, a first portion and asecond portion (FIG. 14).

According to this embodiment, the stacked film 9 over the base mesa 4 adoes not become an independent pattern, but is linked with the stackedfilm 9 at the outer periphery of the region OA1, which facilitates thepeeling of the stacked film 9. FIG. 15 illustrates the state of thestacked film 9 over a plurality of HBT formation regions (blocks).

As illustrated in FIG. 23, when no cutout portions are disposed in thepattern of the collector electrode 9 a, the stacked film 9 over the basemesa 4 a becomes an isolated pattern and cannot be peeled easily (FIG.24). In other words, the longer the perimeter of the pattern of thestacked film 9 to be removed, the more easily it is peeled. When thepattern becomes an isolated D-shaped pattern, its perimeter becomesshort and the stacked film 9 tends to remain.

Provision of the two cutout portions, as in this embodiment, makes iteasy to prevent the retention of the stacked film 9 over the base mesa 4a. For example, when there is only one cutout portion, as illustrated inFIG. 25, the stacked film 9 tends to remain in a region opposite to theone cutout portion.

A gap is formed between the resist film R and the stacked film 9 owingto the peeling force of the stacked film 9 at the outer periphery of theregion OA1, which is a relatively large pattern, and peeling proceedsfurther. The stacked film 9 over the base mesa 4 a can be easily peeledoff when there are two starting points of peeling. In addition, whenthese starting points are located at positions opposite to each other inthe pattern of the base mesa 4 a, the peeling of the stacked film 9 overthe base mesa 4 a becomes even easier (refer to FIG. 15).

These two cutout portions do not necessarily need to exist at oppositepositions, that is, on both sides relative to the center of the HBTformation region. For example, an angle formed by lines connecting twocutout portions and the center of the HBT formation region may be 90° orgreater. Also, the number of cutout portions may exceed 2.

When one of the two cutout portions is disposed at a chord of the regionof the base mesa 4 a, a base extraction interconnect M1 b, which will bedescribed later, can be formed more easily. Moreover, if so, theparasitic capacitance between the base extraction interconnect M1 b andcollector extraction interconnect M1 c (collector electrode 9 a) can bereduced.

As illustrated in FIG. 16, the insulating film 13 a is then removed,followed by the removal of the n type GaAs layer (collector layer) 3 andn⁺ type GaAs layer (sub-collector layer) 2 from the outside of thecollector electrode 9 a by etching, whereby the transistors areelectrically separated from each other. At this time, the n type GaAslayer (collector layer) 3 and n⁺ type GaAs layer (sub-collector layer) 2at the periphery of the backside via electrode 7 v are also removed.

The separation between transistors may be effected by implantation of ap type impurity into the n⁺ type GaAs layer (sub-collector layer) 2outside the collector electrode 9 a (pn separation).

As illustrated in FIG. 17, an insulating film 13 b, such as a siliconoxide film, is deposited by CVD over the substrate 1. Etching of the ntype GaAs layer (collector layer) 3 and the n⁺ type GaAs layer(sub-collector layer) 2 for separation is carried out while leaving theinsulating film 13 a, and the insulating film 13 b may be formed overthe insulating film 13 a.

The insulating film 13 b over the emitter electrode 7, base electrode 8,and collector electrode 9 a are removed to form a connecting hole. Aconductive film, such as a stacked film of molybdenum (Mo), Au and Mo(which will hereinafter be called “Mo/Au/Mo film”), is deposited overthe insulating film 13 b, including the inside of the connecting hole.The Mo/Au/Mo film is etched to form an emitter extraction interconnectM1 e, base extraction interconnect M1 b and collector extractioninterconnect M1 c. At this time, an interconnect M1 v is formed over thebackside via electrode 7 v. These interconnects are defined as afirst-level interconnect. FIG. 18 is a fragmentary plan view of thefirst-level interconnect after its formation. Since the base extractioninterconnect M1 b is disposed over the cutout portion (20 b), the baseextraction interconnect M1 b does not cause unevenness attributable tothe collector electrode 9 a.

As illustrated in FIG. 19, an insulating film 13 c, such as a siliconoxide film, is deposited over the first-level interconnect (M1 e, M1 b,M1 c, M1 v), for example, by CVD. Then, the insulating film 13 c overthe emitter extraction interconnect M1 e is removed to form a connectinghole. A conductive film, such as a Mo/Au/Mo film, is deposited over theinsulating film 13 c, including the inside of the connecting hole. Then,the Mo/Au/Mo film is etched to form an emitter extraction interconnect(second-level interconnect) M2 e. FIGS. 20 and 21 are fragmentary planviews of the second-level interconnect after its formation. Asillustrated in these diagrams, the emitter extraction interconnect M2 eextends even over the backside via electrode 7 v. For example, FIG. 19corresponds to the cross-sectional view taken along a line A-A of FIG.21. Alternatively, the emitter extraction interconnect 2 e may bewidened enough to cover the emitter extraction interconnect M1 e. Thesymbol VH indicates a via hole, which will be described later.

As illustrated in FIG. 22, an insulating film 13 d, such as a siliconoxide film, is deposited over the second-level interconnect (M2 e).

A resistor element and capacitor element are formed in an unillustratedregion over the substrate 1 as needed, and the surface of the substrateis covered with a protecting film.

The backside of the substrate 1 is then polished with the protectingfilm side (element formation surface) down, whereby its thickness isadjusted to 70 to 100 μm. Using an unillustrated resist film as a mask,the substrate 1, n⁺ type GaAs layer (sub-collector layer) 2, n type GaAslayer (collector layer) 3, p type GaAs layer (base layer) 4, n typeInGaP layer (emitter layer) 5 and upper emitter layer 6 over thefirst-level interconnect M1 v are etched to form a via hole VH. Dryetching is employed, for example, as the etching method. Depositsproduced upon dry etching are removed by wet processing. For this wetprocessing, a mixed solution of ammonia and hydrogen peroxide isemployed.

With the first-level interconnect M1 v serving as an etching stopper,the backside via electrode (Wsi) 7 v is also etched. The Mo existingbelow the first-level interconnect (Mo/Au/Mo film) M1 v is also etched.Accordingly, the backside via electrode (WSi) 7 v and Mo exist annularlyaround the via hole VH. In other words, a stacked film of the backsidevia electrode (WSi) 7 v and Mo remains on the side of the via hole VH.

A metal film, such as Au, is formed over the back side of the substrate1, including the inside of the via hole VH, by plating, whereby a backelectrode 40 is formed. This back electrode 40 is brought into contactwith a portion of the Au constituting the first-level interconnect M1 v,so that the contact resistance is reduced. Since the Au itself is a lowresistance material, its use as an interconnect for the connection withthe back electrode 40 is suitable. Alternatively, Au/Mo/WSi orAu/Pt/Timay be used as an interconnect.

In this Embodiment, the backside via electrode 7 v is formed using amaterial, such as WSi, which does not easily react with the n type GaAslayer or n type InGaAs layer constituting the upper emitter layer 6, sothat generation of side etching upon formation of the via hole VH can bereduced. In addition, the backside via electrode 7 v is formed in thesame step with that for forming the electrode (emitter electrode 7, inthis case), which is formed of a material which hardly reacts with the ntype GaAs layer or n type InGaAs layer, constituting the upper emitterlayer 6, so that the number of steps can be reduced.

For example, as illustrated in FIG. 26, the backside via electrode 17 vmay be formed in the same step with that for the collector electrode(Auge/Ni/Au). In this case, however, the n type GaAs layer or n typeInGaAs layer comes in contact with the bottom AuGe layer, and a reactionlayer is formed at the contact site (alloyed).

This reaction layer is apt to be etched with an etchant (for example, amixed solution of ammonia and hydrogen peroxide) used for the wetprocessing, so that side etching appears on the bottom of the via hole.This side etching portion is defined as F. When the back electrode 40 isformed after that, the side etching portion F becomes hollow and thebackside via electrode 17 v and back electrode 40 are easy to peel.

The reaction layer between the bottom AuGe layer and the semiconductoris relatively fragile. Even when wet processing is omitted and a sideetching portion is not formed, the reaction layer is not resistant to amechanical force and the backside via electrode 17 v easily peels.

As illustrated in FIG. 27, the back electrode 40 becomes thin at theside etching portion F, which sometimes leads to disconnection. When theback electrode 40 is thin, an increase in the electrode resistance ordeterioration in electromigration resistance occurs. Thus, thereliability of the back electrode 40 lowers.

In this embodiment, on the other hand, generation of side etching can bereduced and adhesion between the backside via electrode and thesemiconductor can be improved. This leads to an improvement in thereliability of the back electrode.

In this embodiment, WSi is used as a material (non-alloyed material),which hardly reacts with a compound semiconductor layer and exhibitsgood adhesion with a semiconductor, but a refractory metal, a nitride orsilicide thereof can be used alternatively. Examples include refractorymetals such as Ti, W, Ta and Mo and compounds of a refractory metal suchas titanium tungsten (TiW). Nitrides of a refractory metal (for example,TiN) and silicides of a refractory metal (for example, TiSi, TaSi andMoSi) may be used as well.

When the backside via electrode 7 v and the emitter electrode 7 areformed in the same step, a stacked film of Ti/Pt/Au, which has beenstacked in this order, may be used. It is needless to say that thebackside via electrode 7 v and emitter electrode 7 are formed indifferent steps by using different materials.

In this embodiment, a transistor having a pattern shape as describedwith reference to FIG. 14, has been proposed was described. However, theshape is not limited thereto, but a circular pattern, as illustrated inFIG. 28, may be adopted as well.

As described with reference FIG. 18 and FIG. 20, the resistance of theemitter electrode can be lowered by widening the contact region betweenthe emitter extraction electrode M1 e or M2 e and the emitter electrode7. For example, the sheet resistance of the interconnect is 0.04 Ω/□ andthe sheet resistance of the emitter electrode (WSi) is 6 Ω/□.

At the base extraction interconnect M1 b portion, however, an emitterextraction interconnect cannot be formed, and a parasitic resistance ofseveral Ω is connected to the emitter layer in series, which raises theresistance of the emitter electrode 7. In order to lower the resistanceof the emitter layer, it is desired to form a cutout portion on theextraction side of the base electrode, as illustrated in FIG. 14.

A rectangular pattern, as illustrated in FIG. 29, may be employed. Forexample, when a rectangular pattern as illustrated in FIG. 29 isadopted, an emitter electrode 7 and base electrode 8, each having alinear shape, are formed over the base mesa 4 a, and a collectorelectrode 9 a encompassing the base mesa 4 a is composed of a pair ofpatterns having a portion extending in the Y direction and anotherportion extending in the X direction from each end of the formerportion. A contact portion over the base electrode 8 is indicated by C.

In the above-described embodiment, an npn type bipolar transistor wasdescribed, but the present invention may be applied to a pnp typebipolar transistor as well. The bipolar transistor described herein isformed over a GaAs substrate, but another compound semiconductor may beused instead.

The present invention can be widely applied to a semiconductor devicehaving an isolated pattern. The present invention is particularly suitedfor a bipolar transistor with an annular structure, because it tends tohave an isolated pattern.

The present invention can be widely applied to semiconductor deviceshaving a via hole for connecting a back electrode and a surfaceinterconnect. Use of the present invention is particularly suited for aGaAs or InP substrate, because via holes tend to be formed when thesubstrate has a semi-insulating portion.

The HBT in the above-described embodiment has a GaAs substrate and anInGaP layer as an emitter semiconductor layer. The present invention is;also suited for an HBT having a GaAs substrate and an AlGaAs (aluminumgallium arsenide) layer as an emitter semiconductor layer, because thesame electrode material can be used.

The present invention is also suited for an HBT having an InP (indiumphosphide) substrate, because a collector electrode can be made of anAu-containing metal layer by the lift-off method.

The invention made by the present inventors has been describedspecifically based on one embodiment. However, it should be borne inmind that the present invention is not limited to or by the describedembodiment. It is needless to say that the present invention can bemodified within an extent not departing from the scope of the presentinvention.

Advantages available by the representative invention, as disclosed inthe present application, will next be described briefly. The presentinvention makes it possible to improve the properties of a bipolartransistor.

1. A semiconductor device, comprising: (a) a semiconductor substrateincluding a first region, a second region encompassing the first regiontherewith, and a third region encompassing the second region therewith;(b) a collector layer formed in the first region and the second region;(c) a base layer formed in the first region over the collector layer;(d) an emitter layer formed over the base layer; and (e) a collectorelectrode formed in the second region over the collector layer andhaving a first portion and a second portion separated from each other bytwo cutout portions.
 2. A semiconductor device according to claim 1,wherein the collector layer and the base layer are each comprised ofgallium arsenide (GaAs), while the emitter layer is comprised of indiumgallium phosphide (InGaP) or aluminum gallium arsenide (AlGaAs), andwherein the collector electrode is comprised of a film having gold (Au)as a main component.
 3. A semiconductor device according to claim 1,wherein the first region has a substantially circular shape or acircular shape with a portion cut out thereof.
 4. A semiconductor deviceaccording to claim 1, wherein the two cutout portions are arrangedsubstantially symmetrical to the first region.